A 6-3 Supreme Court majority on Friday struck down President Trump’s sweeping emergency tariffs (Learning Resources v. Trump) in a monumental vindication of the Constitution’s separation of powers. You might call it the real tariff Liberation Day. It’s hard to overstate the importance of the Court’s decision for the law and the economy. Had Mr. Trump prevailed, future Presidents could have used emergency powers to bypass Congress and impose border taxes with little constraint. As Chief Justice John Roberts explains in the majority opinion, “Recognizing the taxing power’s unique importance, and having just fought a revolution motivated in large part...